In recent years, the importance of thin films used as a barrier film or the like has been increasing in the field of electronics. In the case of, for example, an organic EL element, a water barrier film high in waterproof property is formed on its element body, which is formed on the front or rear surface of a glass substrate, in order to protect the element body. Recently, instead of the glass substrate, a resin film substrate has been used in order to make organic EL elements thin or give flexibility thereto. In this case, the waterproof property of the resin film substrate is not high; thus, water barrier films are formed directly not only on the element body, which is formed on the front surface of the substrate, but also on the front surface and the rear surface of the resin film substrate.
As the water barrier film(s) referred to herein, for example, a Si sputtering film or a silicon oxide based or silicon nitride based sputtering film is known. When sputtering films are compared with vapor-deposited films, the sputtering films give a lower film forming rate, but it is said that the sputtering films have a denser film structure, a higher adhesiveness to objective substances, and a higher film quality. For this reason, it is said that in the field of electronics, such as organic EL elements, which are required to have a high water barrier property and the like, the sputtering films are preferred.
As is well known, in sputtering, an atmospheric gas is made into plasma, and the ionic particles in the gas are electrically or magnetically accelerated and collided with the surface of a target material. By this collision, material particles are discharged from the surface of the target material and are caused to adhere onto a surface of an objective substance, thereby forming a thin film of the target material on the surface. As the target material, Si or an oxide thereof is used (see Patent Documents 1 and 2).
Patent Document 1: JP-A-2002-275628
Patent Document 2: JP-A-2004-176135
About the atmospheric gas, an inert gas such as an argon gas is basically used for electric discharge; however, in the case of reactive sputtering, a mixture of the gas and a reactive gas such as an oxygen gas or a nitrogen gas is used to form an oxide-based, nitride-based or oxynitride-based sputtering film. Although the nitride film of Si is opaque, the transparency thereof increases as the oxidation degree rises. On the other hand, the barrier property becomes better as the nitriding degree becomes higher. For these reasons, the oxynitriding degree of the sputtering film is adjusted by the composition of the reactive gas. For reference, it is said that a SiN film is opaque but high in barrier property while a SiOx film is transparent but relatively low in barrier property.
However, it has been proved that conventional sputtering films have a problem described below on the basis of sputtering target materials therefor in, for example, organic EL elements, in particular, organic EL elements wherein a resin film substrate is used.
As described above, by use of oxygen and nitrogen as reactive gases, the barrier property and the transparency of the sputtering film are adjusted. When a sputtering film having a specific barrier property and transparency is formed in the case that the target material therefor is Si, a fall in the flexibility of the sputtering film becomes a problem. Moreover, it is feared that an objective substance wherein the film is to be formed is damaged since a large amount of oxygen is required for the reactive gas.
Specifically, in the case of a sputtering film for an organic EL element, it is necessary to form the film on its element body on the rear surface of its substrate. In order to relieve damages on the element body, a low temperature is required and further the film-formation is required to be carried out in a low oxygen atmosphere. The sputtering film is also required to have a high flexibility. However, in the case that the target material is Si, the above-mentioned requirements are not easily satisfied since the amount of oxygen in the atmospheric gas becomes large.
Sputtering films for organic EL elements are required to have a by far higher water barrier property than those in the prior art. Specifically, when the property is represented by, for example, water vapor permeability, the sputtering films are required to have a super high level water barrier property of less than 0.01 g/m2 per day. In the case that the target material is Si, the water barrier property of the sputtering film therefrom is relatively good.
About Si, described above, electroconductivity is easily given thereto by doping. For this reason, DC sputtering, wherein a direct current is applied to between a target and an objective substance, can be realized. Thus, there is produced an advantage that the film forming rate is considerably high although this manner is one out of sputtering manners.
In the case that against such a Si target material a SiO target material is used, the amount of oxygen as a reactive gas can be decreased so that damages of an objective substance wherein a film is to be formed can be relieved. Moreover, SiO has an advantage that SiO is easily powder-sintered so as to be worked into a target material with ease. However, SiO targets, which are different from Si targets, are electrically insulators; thus, the SiO targets cannot be subjected to the above-mentioned DC sputtering. As a result, it is indispensable to use RF (high frequency) sputtering, which is lower in efficiency than the DC sputtering. Accordingly, a remarkable fall in the film forming rate becomes a problem.
Moreover, in the actual circumstances, the sputtering film obtained in the case of using a SiO target does not have such a flexibility that the film can be used on a thin resin film substrate in the same manner as the sputtering film obtained in the case of using a Si target.
For reference, SiO2 is low in reactivity, and is not easily adjusted, in composition, toward a SiOx or SiON film; therefore, SiO2 is unsuitable as a target material in reactive sputtering.